This invention relates generally to wireless communication systems using passive networks, and more particularly, to a planar variable power divider with low passive intermodulation for use on printed circuit boards to convert a single input RF signal into two output RF signals of constant phase throughout the adjustment range but with variable amplitudes as a function of movement of a single phase shifter that is part of the variable power divider.
A large class of microwave components can be formed by combining two phase shifters and two fixed power dividers (combiners). The fact that both of these components may be made to operate over broad frequency bands at relatively high RF power levels has made this general structure useful in constructing variable power dividers, switches, and fixed circulators for active electronic warfare and beamforming in antenna applications for communication satellites and radar.
General Discussion of Conventional Technology
FIGS. 1 through 5 illustrates five conventional configurations incorporating two phase shifters and two fixed power dividers to function as variable power dividers and switches. FIGS. 1 through 4 illustrates networks having four ports and FIG. 5 illustrates a network having three ports. Other networks exist having three or four ports, and networks having greater numbers of ports can be realized with fixed power dividers having greater numbers of ports and additional phase shifters. Networks having greater numbers of ports can be realized using networks having three or four ports as building blocks. The three or four port configurations presented in FIGS. 1 through 5 can be realized as either switches (having two states) or variable power dividers (having a continuum of states).
In the case of a switch, only two values of phase shift (and therefore two states) are available: those phase settings corresponding to state 0 and state 1. For the variable power divider, the setting of phase shifters xcfx861 and xcfx862 may vary continuously over a predetermined range of values. The use of phase shifter pairs having unlike insertion phases will result in different phase values for state 0 and state 1 than the ones shown. The use of phase shifters with nonreciprocal phase properties will result in different phase values corresponding to the forward (transmit) or reverse (receive) signal propagation through the device. Four port circulators can be made using the configurations in FIGS. 1 through 4 comprised of four external ports with fixed phase states when the phase shifters have nonreciprocal phase properties.
The configuration illustrated in FIG. 1 uses a zero degree/one-hundred-eighty degrees hybrid power divider and a quadrature (zero degree/ninety degrees) hybrid power divider. The output voltage signals, b3 and b4, at Ports 3 and 4 described by the equations in FIG. 1 correspond to an input signal at Port 1. The input signal at Port 1 provides in-phase signals of equal amplitude to the variable phase shifters xcfx861 and xcfx862. Ideally no signal appears at Port 2 when a signal is applied to Port 1, and Port 2 can be described as the xe2x80x9cisolated portxe2x80x9d for signals applied to Port 1. Similarly, a signal applied to Port 2 does not appear at Port 1. The phase difference, xcex94xcfx86=xcfx861xe2x88x92xcfx862, is the controlling parameter for the output signal amplitudes at Ports 3 and 4 and the sum of the two phase values can vary the output signals phase. The sum of the two phase values must be equal to a constant phase value throughout the range of adjustment for the output signals to have a constant phase value.
Simultaneously altering the phase values in a complementary fashion can accomplish variable power divider output signal amplitude variation while maintaining a relatively constant output signal phase values throughout the range of adjustment. The variable power divider function of varying the output signal amplitudes can be accomplished by varying the phase value of one phase shifter while the phase of the other phase shifter remains at a fixed value. The output signals phase values are substantially a constant quantity only when the phase quantity (xcfx861+xcfx862) is substantially equal to a constant value throughout the range of adjustment.
The range of phase values to control the signal amplitudes between the switch states for the configuration illustrated in FIG. 1 is ninety degrees. The table in FIG. 1 identifies the phase values for xcfx861 and xcfx862 where xcex94xcfx86=xe2x88x9290 degrees for switch State 0 and xcex94xcfx86=+90 degrees for switch State 1. State 0 corresponds to the condition where ideally all of the available signal input to Port 1 appears at Port 4. State 1 corresponds to the condition where ideally all of the available signal input to Port 1 appears at Port 3. Values of the xcfx861 and xcfx862 phase values in the table greater than zero represents a greater phase delay relative to the zero degree value for signals input to phase shifters xcfx861 and xcfx862 having identical phase values.
In other words, xcfx861=0 degrees and xcfx862=90 degrees is a condition where the signal output from xcfx862 is delayed 90 degrees relative to the signal output from xcfx861. In other words, xcfx861=0 degrees and xcfx862=90 degrees is a condition where the signal output from xcfx862 lags 90 the signal output from xcfx861 by 90 degrees. The insertion loss of the phase control devices can be minimized when the phase control devices have the minimum range of phase adjustment corresponding to the desired range of amplitude adjustment.
The configuration of FIG. 5 having three external ports is the same as FIG. 1 except the input divider does not have the isolated Port 2 and the input divider consequently is a reactive type power divider and not a hybrid power divider. The operation of the configuration in FIG. 5 is identical to that of FIG. 1.
The configuration illustrated in FIG. 2 uses two quadrature hybrid power dividers as compared to the mixed hybrid configuration illustrated in FIG. 1. The range of phase values to control the signal amplitudes between the switch states in FIG. 2 is one-hundred-eighty degrees and the insertion loss of the phase shifters can be greater than the configuration in FIG. 1.
The configuration illustrated in FIG. 3 uses zero degree/one-hundred-eighty degrees hybrid power dividers rather than mixed hybrids (FIG. 1) or quadrature hybrids (FIG. 2). In this configuration, one-hundred-eighty degrees of phase shift is required of each phase shifter. The output signals at Ports 3 and 4 have phase values that are different by ninety degrees.
The configuration of FIG. 4 is the same as FIG. 2 with an additional fixed phase delay, xcfx860, and a length of transmission line, L, so the two signal phases coincide at the input to the respective variable phase shifters xcfx861 and xcfx862. This configuration has the same overall functionality as the configuration in FIG. 1.
Specific Discussion of Conventional Technology
U.S. Pat. No. 4,485,362 to Campi et al. teaches a three-port, variable microwave stripline power divider that has a variable output over a wide range at one output without appreciably changing the power output at the other output, but which requires electronic patch devices and circuitry to vary the power split.
U.S. Pat. No. 5,473,294 to Mizzoni et al. teaches a planar variable power divider but which requires use of two quadrature hybrids and two variable phase shifters, and uses waveguide, not microstrip technology, and requires use of two sliding mechanisms to close the four hybrid output circuits. The block diagram for Mizzoni et al. conforms to FIG. 4 knowing that the quadrature hybrids with sliding shorts as described by Mizzoni et al. are well known in the art as being two port phase shifters.
A variable power divider operated in reverse becomes a variable power combiner whereby two input signals are combined into a single output signal at a predetermined power level. Such a combiner is as taught in U.S. Pat. No. 6,069,529 to Evans, where a variable power combiner is used as a redundancy switch to provide amplified signal backup in the event of a failed first amplifier. However, it uses a waveguide path, requires active amplifier circuitry, and a mechanical apparatus within the hybrid comprising a movable coupling plate that is replaceable with a metal wall. Such a design is costly and adds complexity to its manufacture. The design is also characterized by reduced reliability, while also being limited to waveguide medium applications.
Japanese Patent No. 4000902 by Asao et al. teaches a planar variable power distributor implemented in stripline technology having a block diagram that conforms to FIG. 1 with the exception that it has two isolated ports instead of the one isolated port (2) in FIG. 1. The fixed input divider is a xe2x80x9crat-racexe2x80x9d or xe2x80x9cringxe2x80x9d hybrid comprising five ports and the in-phase port is used as the input (1) to the variable power distributor. The two isolated ports are terminated with absorbing loads. The parallel lines between the input in-phase hybrid divider and the quadrature divider are covered in part with two diamond-shaped dielectrics.
Moving the dielectrics in tandem in the direction transverse to the direction of the parallel lines results in differential and complementary phase shifts on the two lines. The design has varying amounts of dielectric material in close proximity to fixed width transmission line conductors. The impedance of the transmission lines will change along with the phase shift unless some other geometric parameter such as separation distances between the two ground planes and the transmission lines simultaneously vary.
Problems in Conventional Art
The variable power dividers of the conventional art have required more than one phase shifter to achieve output signals with substantially constant phases throughout the adjustment range, have been limited to use with the more costly waveguide transmission medium, or have relied on use of complex mechanical apparatus as part of the hybrid network. Even the one stripline power divider to Campi et al. requires the connection of various contact points between a patch member and ground to effectuate discreet power splits between two outputs, which themselves are required to be two planar patch members.
Accordingly, a need exists in the art for a variable power divider in which the output signals can be easily controlled, either locally or remotely, by a simple, single movable part. A need further exists for a variable power divider suitable for planar construction on a printed circuit board using microstrip or strip line transmission lines, having a single input port and two output ports where the two output signals are variable in amplitude and with phases that are substantially a constant quantity throughout the adjustment range, and the constant output signal phases are either substantially equal or different by a fixed value.
Another need exists for a variable power divider in which the variable amplitudes of the output signals is accomplished by means of a single moveable part that varies the phase of the input signal in two signal paths, and that single moveable part may be operated locally or remotely.
There is a further need in the art to provide a variable power divider that is suitable for planar construction on a printed circuit board and used with microstrip or stripline transmission paths on the printed circuit board.
And lastly, another need exists to produce a variable power divider that is easily constructed, of low cost, adaptable to common printed circuit board manufacturing techniques, highly reliable by its simplicity of component parts and easily variable and repeatable signal outputs.
The present invention solves the aforementioned problems with a variable power divider and method that can vary the RF power between ports in a high power and multi-carrier RF environment, such as is used in controlling signals sent and received in a base station antenna. The variable power divider can comprise a single-control phase shifter and a hybrid power divider.
The single-control phase shifter is a three-port device having a single input port and two output ports. The single-control phase shifter of the present invention is reciprocal and therefore, a circulator function, as taught in the conventional art, cannot be realized with this invention. The single-control phase shifter can further comprise a variable adjuster that can change or adjust the phase between two RF signals. Specifically, the variable adjuster can change the phase between two RF signals propagating along two electrical paths by changing the electrical lengths of the paths relative to each other. In this way, a sum of a first phase of a first RF signal and a second phase of a second RF signal can be maintained to be substantially equal to a constant as measured at the output ports of the three port phase shifter.
The single-control phase shifter can propagate RF signals between contactless conductive structures in order to substantially reduce passive intermodulation. Specifically, the variable adjuster of the single-control phase shifter can capacitively couple RF signals between non-contacting conductive structures. The variable adjuster can comprise a moveable first electrical path that can be rotated and capacitively coupled to various positions along a second electrical path that propagates received RF signals in opposite directions relative to one another.
However, the present invention is not limited to this specific mechanical structure of a first electrical path that can be rotated and capacitively coupled to various positions along a second electrical path. Other phase shifter structures can include, but are not limited to, capacitively coupled sliding sleeves, moving dielectrics in tandem, waveguides, and other similar structures that have three ports and can impart phase shifts between RF signals such that a sum of the phase shift values substantially equals a constant quantity throughout the range of adjustment.
Meanwhile, the hybrid power divider is a four port device having two input ports and two output ports and the hybrid power divider is reciprocal. The hybrid power divider manipulates both the phase and amplitude of the RF signals received at its input ports. The hybrid power divider can substantially isolate the input RF signal flow between the input ports. Since very little signal flow occurs between the two input ports, predominate RF signal flow in the hybrid power divider is from the input ports to the output ports.
The RF signal amplitudes at the two output ports of the phase shifter corresponding to the RF signal from one input port usually have a substantially equal amplitude. The RF signal phase values at the two output ports of the hybrid power divider corresponding to the RF signal from one of the input ports of the hybrid power divider can differ by substantially ninety or one-hundred-eighty degrees. There can be two signals at each output port of the hybrid power divider when there is one signal applied to each of the input ports of the hybrid power divider.
The addition of the two RF signals at each output port of the hybrid power divider can provide a resultant RF signal with amplitude and phase that is dependent on the relative signal amplitudes and phases of the input RF signals. The phase of each input RF signal can be adjusted such that each phase of a respective output RF signal is substantially equal to a constant value throughout the range of adjustment. Furthermore, the phase of each input RF signal can be adjusted such that each phase of a respective output RF signal is substantially equal to a constant value while the relative amplitude values of the output RF signals are varied. The variable power divider of the present invention is specific to the output signal phase values that are substantially a constant quantity throughout the adjustment range of the phase shifter.
According to one exemplary embodiment, the phase of a first RF signal at a first output port and the phase of a second RF signal at a second output port of the hybrid power divider are substantially equal. According to another exemplary embodiment, a phase difference of substantially a constant amount exists between a first RF signal at a first output port of the hybrid power divider and a second RF signal at a second output port of the hybrid power divider.
Both the single-control phase shifter and the hybrid power divider can comprise substantially planar structures that are suitable for high-speed manufacturing environments that can substantially reduce manufacturing costs. Specifically, both the single-control phase shifter and the hybrid power divider can be made from substantially planar printed circuit board materials.
The output ports of the variable power divider can be coupled to various devices. According to one exemplary aspect of the invention, the variable power divider output ports can be coupled directly or indirectly to antenna elements of an antenna array to vary an antenna radiation characteristic. According to another exemplary aspect of the present invention, the variable power divider can be coupled to two RF signal paths and operated with two states and function as a RF switch to route the RF input signal to substantially one output port and to the respective signal path. According to another exemplary aspect of the present invention, one of the output ports can be coupled to a RF power absorbing element. In this way the variable power divider can function as a variable power attenuator since one output port can dissipate RF energy usually in the form of heat while the other output port propagates the RF energy to another device that conserves RF energy such as an antenna.
The phase shifter of the variable power divider can be moved with an actuator that can comprise an electromechanical device such as an electric motor. The actuator can be coupled to a remote controller through a control link that may comprise a wireless or cable type of communications medium. The remote controller can comprise a computer running software that determines how the much the phase shifter should be adjusted in order to control the power distribution at the outputs of the hybrid power divider.